Make use of HAIKU_BOOT_PLATFORM to build EFI boot platform.

Moving from proof of concept EFI building to a proper boot platform.
 * Fix missing paranthesis in if case in BuildSetup.
 * Adding efi setup of compiler flags in BuildSetup
 * Remove efi compiler flags from efi Jamfile.

You now build haiku.efi by passing -sHAIKU_BOOT_PLATFORM=efi to jam:
  jam -sHAIKU_BOOT_PLATFORM=efi haiku.efi

Coming up, more work to be a proper boot platform..